1. Field of the Invention
The present invention relates to an optical disc reproduction apparatus for optically reading a signal from an optical disc serving as a recording medium to reproduce data and, more particularly, to a technique of automatic adjustment of a servo system performed before reproduction of data in an optical disc reproduction apparatus
2. Description of the Prior Art
It is well known to use an optical disc as a recording medium from which signals can be optically read, for example, a CD (Compact Disc) (including a CD-DA (Compact Disc Digital Audio), a CD-ROM (Compact Disc Read Only Memory), a CD-R (Compact Disc Recordable), a CD-RW (Compact Disc Rewritable)), a DVD-ROM (Digital Video Disc Read Only Memory), a DVD-RW(Digital Video Disc Rewritable), and the like.
Conventionally, in optical reproduction apparatuses for reproducing data from such optical discs, automatic adjustment to the servo systems is carried out before the reproduction of data as follows. That is, once an optical disc is loaded on an optical reproduction apparatus, an optical pickup for reading a signal from the optical disc is first moved in an inner radial direction of the optical disc. The position of the pickup in the radial direction is identified by a limit switch which is adapted to be turned on when the pickup reaches a predetermined position at an inner periphery. Thus, the pickup is located on a lead-in area formed at the inner peripheral side of the optical disc. Signals are then read from the lead-in area by means of the pickup irradiating the optical disc with laser light. Simultaneously, in order to bring the pickup to a state in which signals are accurately read out with stability, there are provided automatic adjustments to characteristics including an offset, a phase and a loop gain value in various servo control such as focus servo control of the pickup, servo control of rotation of the optical disc, tracking servo control thereof, and the like. After such initial adjustments, reproduction of data from the optical disc is started.
In some lead-in areas of DVD-ROM discs, there is an area called Burst Cutting Area (BCA) with signals radially recorded thereon, which area has a lower refractive index than that of other areas having signals recorded in the form of normal pits. In this case, appropriate automatic adjustment is not available in the BCA part of the lead-in area within the DVD-ROM. In addition, parts other than the BCA part in the lead-in area of the DVD-ROM are limited or restricted in a radial direction due to the existence of the BCA. This makes it difficult to accurately position the optical pickup within parts other than the BCA in the lead-in area when using the conventional automatic adjustment technique.
Provided that both CDs and DVD-ROMs are capable of being reproduced in a single optical disc reproduction apparatus, the following problem will arise. That is, the lead-in area of the CD and the lead-in area of the DVD-ROM partially overlap in a radial direction. It has been suggested to read out signals and perform automatic adjustment by disposing the optical pickup within a range where the CD lead-in area overlaps the DVD-ROM lead-in area. The aforesaid BCA range of the DVD-ROM, however, also partially overlaps the lead-in area of the CD in the radial direction. Accordingly, when carrying out the described automatic adjustment simply by positioning the pickup within the range of the CD lead-in area that overlaps the DVD-ROM lead-in area and reading signals therefrom, there is a possibility that signals are read from the BCA, resulting in inappropriate adjustment.
It should be noted that there is provided an area called Narrow Burst Cutting Area (NBCA) within a lead-in area in some DVD-RW discs with signals radially recorded thereon and which has a lower refractive index. Also, there is a possibility that this NBCA is located within a range of the DVD-RW lead-in area that overlaps the lead-in area of the CD or the DVD-ROM in the radial direction. Therefore, even in the DVD-RW, the above-mentioned problem will arise as in the case with the DVD-ROM.